This invention relates to a method of depositing carbon films which are used to protect magnetic media.
Metallic magnetic thin film disks used in memory applications typically comprise a substrate material which is coated with a magnetic alloy film which serves as the recording medium. Typically the recording medium used in such disks is a cobalt-based alloy such as CoNi, CoCr, CoCrNi, CoPt and CoNiPt which is deposited by vacuum sputtering as discussed by J. K. Howard in "Thin Films for Magnetic Recording Technology: A Review", published in Journal of Vacuum Science and Technology in January 1986, incorporated herein by reference.
Usually, it is necessary to protect such magnetic recording media by sputtering a protective overcoat such as a carbon overcoat. An example of such a sputtered carbon overcoat is described by F. K. King in "Datapoint Thin Film Media" published in IEEE Transactions on Magnetics in July 1981, incorporated herein by reference. It is also known to provide a carbon film containing hydrogen by sputtering the carbon film in the presence of a hydrogen-argon gas mixture as described in U.S. Pat. No. 5,045,165, issued to Yamashita and incorporated herein by reference.
Carbon targets used for sputtering of carbon films generally consist of nearly pure graphite. The targets are electrically conductive. The target materials are readily available from many vendors world wide.
Thin film magnetic disks are typically manufactured in a large inline sputtering machine as described in the U.S. Pat. No. 5,045,165. To increase productivity and reduce manufacturing costs, it is important to operate the sputtering machine continuously with high utilization. There are several factors which limit the continuous operation of the sputtering machine. First, the targets are eventually used up, and the machine must be opened to replace them. Second, the protective shields which direct the sputtered species to the substrate become coated, and this coating will eventually start to flake. Excessive flaking from the shields contributes to increased defects on the disk surface. Therefore, the shields must be periodically replaced and cleaned. The third factor is that the carbon target surface develops small nodular growths, generally known as "warts" or "mushrooms", during continuous sputtering operation. These nodules degrade the sputtering efficiency of the target. The nodules form over the region where the sputtering actually takes place on the target, and the region where the nodules form does not sputter. To make up for the loss in sputtering rate due to the nodules, the power input to the target must be increased to increase the sputtering rate from the target areas which are not yet covered by nodules. However, the power cannot be increased indefinitely because the cathode often becomes unstable at very high power. Target cooling also becomes a problem at high power input. Often, nodule formation on the carbon target forces an early shut down of sputtering machine, during which the sputtering machine is vented to air and the carbon target surface is cleaned to remove the nodules. Sometimes the target is replaced with a new target. The time needed to replace or clean the target results in a loss of utilization of the sputtering machine. Since the sputtering machine is typically the most expensive capital equipment used in making the disks, a loss of utilization adds to the cost of the disks.
Nodules are described by Chen et. al in "Surface-Defect Formation in Graphite Targets During Magnetron Sputtering", Journal of Vacuum Science and Technology July/August 1990, incorporated herein by reference. To date, the mechanism of nodule formation is not well understood, nor is there a satisfactory means of avoiding the nodules. It is generally believed that nodule formation is related to the purity of the carbon target, or to the placement geometry of the structures around the cathode, such as the ground shields around the cathode. We have found however, that nodules can form in many types of carbon targets regardless of their purity level. Cathode construction and geometry has some effect, but generally nodules form readily in most cathode types which we have studied. The nodules occur exclusively in DC magnetron sputtering of carbon. It is possible to sputter carbon by RF and RF/magnetron sputtering, but because the sputtering rate is much lower than that of DC magnetron sputtering, RF or RF/magnetron sputtering is generally not used in production machines. A higher sputtering rate translates to higher throughput for the disks, which increase the productivity of the sputtering machine.
During planar DC magnetron sputtering, sputtering occurs over a region where a magnetic and electric field from a magnet placed under the target is horizontal with respect to the target surface. Typically, this sputter region is a semi-circular area over the target surface, and it is commonly referred to as a "race-track" due to its shape. The construction of planar DC magnetron cathodes is described in a chapter by Robert K. Waits in Thin Film Procsses, edited by John L. Vossen and Werner Kern, Academic Press, Inc. (1978), incorporated herein by reference. The nodules form in the race-track. Typical nodules are anywhere from few millimeters in size, up to 1 to 2 cm across. The height of growth above the target surface can be many millimeters. Cross sectional analysis of the nodules reveal several key features as shown in FIG. 1. At the base, there is a region (21) of very hard and dense glassy material which forms beneath the original surface of target (22). X-ray diffraction analysis of the material in region (21) reveals it to be amorphous, as opposed to the target material which is crystalline graphite. The glassy material is electrically resistive. Above the glassy phase, nodular or whisker-like growth occurs. The nodular growths (23) are fairly delicate and can be easily blown away by strong blast of air. The nodular growths (23) are also amorphous as analyzed by X-ray diffraction.
In order to describe how and why the nodules form, some description of the sputtering process itself is desirable. Sputtering is achieved by placing the target in a low pressure argon atmosphere and applying a large negative bias voltage to the target. The target surface emits electrons into the argon. These electrons are accelerated by the bias voltage, and bombard the argon atoms. The argon atoms are then stripped of an electron and become positively ionized. At the target surface, the positively charged argon atoms accelerate toward the target which, as mentioned above, is negatively biased, and bombard the target surface, thereby ejecting from the target the material to be sputtered. Additional electrons can be ejected, called secondary electrons, from the target surface as a result of argon bombardment which then can ionize more argon atoms. The process becomes "self-sustaining", when a plasma of ionized argon atoms forms above the target surface. In a planar DC magnetron sputtering, the electrons are confined in a narrow area above the target surface by a magnetic field. The magnetic field causes electrons to take a longer helical path above the target, which increases the probability of further ionizing collisions with the argon atoms. The plasma thus formed is much denser than can be achieved by any other means, and it is possible to obtain very high sputtering rates.
One problem with DC sputtering is that if an insulating material deposits on the target surface, it can quickly develop a positive charge and prevent argon from bombarding it, so that it will not sputter. Worse yet, if the charge cannot be dissipated, then sufficiently high voltage can develop in the insulating material so that dielectric breakdown can occur. Large current can flow through the insulating material and the target in such a case, and it can damage the power supply and the target. Modern DC power supplies used for sputtering have features which suppress the damage that arcing can cause by momentarily shutting down the power to allow the charge to dissipate. One of the difficulties in the power supply design is the ability to distinguish large arcs which are damaging, from the smaller arcs which are not damaging. Generally, the power supplies will ignore arcing from localized dielectric breakdowns, and will shut down only in the event of more catastrophic arcing which is caused by flakes or larger metal shorting the target to ground. If the target itself is electrically conductive but the sputtered species is non-conductive, the situation becomes worse. This situation occurs in some reactive sputtering systems, where a metal target is sputtered in the presence of oxygen or nitrogen to produce metal oxides or nitrides. The sputtered film is usually nonconductive, and arcing can occur when the sputtered species backsputter onto the target surface and build up over the sputtering region of the target.
Backsputtering is a function of the material being sputtered, process pressure, geometry of the cathodes and power input.
Nodules are believed to form on carbon targets as follows: During sputtering, some local inhomogeneity or small foreign particle on the target surface causes local arcing to occur. The target is under large negative potential, and foreign particles or local inhomogeneities on the target surface which are insulating can develop a positive charge. If the build-up of charge is sufficiently large, dielectric breakdown can occur through the foreign particle or inhomogeneity and a large current can flow through it. Localized arcing apparently causes a very intense temperature rise, and the adjacent graphite transforms into hard glassy carbon. This hard glassy carbon is quite resistive and does not sputter as well as the surrounding graphite material. As sputtering is continued, a backsputtering process starts to deposit sputtered carbon above and around the glassy phase. This backsputtered carbon defines the nodule's characteristic whisker-like appearance. The backsputtered carbon film is generally quite resistive as well, so it does not resputter once it starts to grow on the carbon surface. Additional arcing can occur over the growth, and the process can repeat itself many times. Eventually, the whisker growth can take on a very complex shape as repeated arcing and growth takes place. Because the nodules form over the region where the sputtering occurs, overall sputtering efficiency (deposition rate divided by the target area) decreases as more target area is taken out of the sputtering process by the nodule formation.
In a typical continuous sputtering process, the substrate to be coated passes by the target during a fixed interval of time. When the sputtering efficiency decreases, the power density to the target is increased to make up for the loss. The smaller area available for sputtering combined with the power increase to the target increases the voltage developed at the target. A higher voltage causes a higher rate of arcing which in turn causes even more nodules to form. The intensity of arcing becomes larger as well, so that the power supply begins to detect it and starts to shut itself down to protect itself. Eventually the frequency of arcing becomes sufficiently high so that sputtering cannot be continued. At this point, the chamber must be opened and the target surface must be cleaned.
Sometimes the formation of nodules over the target surface is not evenly distributed. In such cases, the deposition uniformity over the disk surface becomes worse, and again the system must be opened to clean the target surface or replace it. The formation of nodules and subsequent additional arcing can cause pieces of nodules to fly off the target surface and deposit on the disk surface. This causes parts of the disk to have no protective carbon coating, and it is detrimental to the performance of the disk. If the carbon target surface is scraped clean of all the nodules, then the target can be returned to its original condition.
It is known that deposition of carbon in the presence of a large amount of hydrogen improves the wear resistance of the carbon film, as described in U.S. Pat. No. 5,045,165. In such a process, the deposited film has electrical resistivity which is much larger than a film deposited without the hydrogen. In the '165 patent, it is stated that the electrical resistivity of a 300 .ANG. thick film increased from 500 .OMEGA./square to more than 20 M.OMEGA./square by adding hydrogen during sputtering. During carbon deposition using hydrogen, the frequency of arcing and the amount of nodules forming over the target dramatically increases. This is presumably because the backsputtered species which starts to redeposit on the target is much more electrically resistive, and this leads to more arcing. Therefore, in order to deposit a hydrogenated carbon film efficiently and productively during manufacturing, arcing must be suppressed and nodule formation prevented.
There are other sputtering methods which are less sensitive to the effect of an insulating layer on the target surface. RF sputtering can sputter insulators without causing arcing, and this largely eliminates the formation and growth of nodules on carbon target surfaces. RF sputtering is described in the chapter by J. L. Vossen and J. J. Cuomo in the above-mentioned Thin Film Processes, incorporated herein by reference. The oscillation frequency used in typical RF sputtering is between 5 and 30 MHz. The industry standard is generally 13.56 MHz. In RF sputtering, the electrons in the glow space oscillate with sufficiently high energy to produce ionization of argon atoms without depending on the secondary electrons. Because the RF voltage can be coupled through any kind of impedance, it is possible to sputter literally any kind of material, including insulators. One key disadvantage of RF sputtering is that a complex matching network must be provided close to the cathode to match the output impedance of the generators, which is typically 50 .OMEGA.. Furthermore, the sputter rate of carbon by RF sputtering is many times lower than by DC magnetron sputtering. Because RF sputtering generates plasma at some distance away from the target and nearer to the substrate, there is substantially more chance of substrate heating by electron bombardment compared to DC magnetron sputtering at a comparable power input. The sputter rate can be increased by supplying RF to the magnetron cathode in which case the sputtering is called RF/magnetron sputtering. The rate can be increased, but not to the level that can be achieved with DC magnetron sputtering, and the matching network is still needed. Also, film uniformity is generally much more difficult to achieve with RF/Magnetron sputtering method. (see page 164, Vossen and Kern).
Alternatively, low frequency RF sputtering in the range of several hundred kilohertz can be used. This greatly simplifies the matching network but the sputter rate is still low so it is not practical.
Pulsed DC sputtering is another method of deposition which attempts to alleviate the problem of arcing. It is often used in reactive sputtering systems. Pulsed DC sputtering uses short pulses of DC voltage separated by short intervals during which the cathode voltage goes to zero. During the "off" period, the charge on any insulating material that developed over the target is allowed to dissipate, thereby preventing catastrophic arcing. Pulsed DC sputtering does not prevent the insulating material from depositing on the target surface in the first place, so eventually the target must be replaced or cleaned.